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参数资料
| 型号： | LTC3867EUF#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 22/36页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 24-QFN |
| 产品培训模块： | LTC3867 Synchronous Step-Down DC/DC Controller |
| 标准包装： | 91 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1.2MHz |
| 占空比： | 98% |
| 电源电压： | 4 V ~ 38 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 24-WFQFN 裸露焊盘 |
| 包装： | 管件 |
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LTC3867
APPLICATIONS INFORMATION
V
+ V
–
V OUT
V IN
Synchronous Switch Duty Cycle = ? IN
( I MAX ) 2 ( 1 + δ ) R DS(ON) +
P MAIN =
? ? V INTVCC – V TH(MIN)
V TH(MIN) ? ?
V IN
MILLER EFFECT
V GS
a b
+ DS
Q IN V GS
C MILLER = (Q B – Q A )/V DS –
3767 F09
Figure 9. Gate Charge Characteristic
across the current source load. The upper sloping line is
due to the drain-to-gate accumulation capacitance and
the gate-to-source capacitance. The Miller charge (the
increase in coulombs on the horizontal axis from a to b
while the curve is flat) is specified for a given V DS drain
voltage, but can be adjusted for different V DS voltages by
multiplying the ratio of the application V DS to the curve
specified V DS values. A way to estimate the C MILLER term
is to take the change in gate charge from points a and b
on a manufacturer’s data sheet and divide by the stated
V DS voltage specified. C MILLER is the most important se-
lection criteria for determining the transition loss term in
the top MOSFET but is not directly specified on MOSFET
data sheets. C RSS and C OS are specified sometimes but
definitions of these parameters are not included. When the
controller is operating in continuous mode the duty cycles
for the top and bottom MOSFETs are given by:
Main Switch Duty Cycle =
? V – V OUT ?
?
? V IN ?
The power dissipation for the main and synchronous
MOSFETs at maximum output current are given by:
V OUT
V IN
? ? 2 ? ?
( V IN ) 2 ? I MAX ? ( R DR ) ( C MILLER ) ?
? 1 1 ?
? + ? ? f
where δ is the temperature dependency of R DS(ON) , R DR
is the effective top driver resistance (approximately 2Ω at
V GS = V MILLER ), V IN is the drain potential and the change
in drain potential in the particular application. V TH(MIN)
is the data sheet specified typical gate threshold voltage
specified in the power MOSFET data sheet at the specified
drain current. C MILLER is the calculated capacitance using
the gate charge curve from the MOSFET data sheet and
the technique described above.
Both MOSFETs have I 2 R losses while the topside N-channel
equation includes an additional term for transition losses,
which peak at the highest input voltage. For V IN < 20V,
the high current efficiency generally improves with larger
MOSFETs, while for V IN > 20V, the transition losses rapidly
increase to the point that the use of a higher R DS(ON) device
with lower C MILLER actually provides higher efficiency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
The term (1 + δ ) is generally given for a MOSFET in the
form of a normalized R DS(ON) vs temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The optional Schottky diodes conduct during the dead
time between the conduction of the two large power
MOSFETs. This prevents the body diode of the bottom
MOSFET from turning on, storing charge during the dead
time and requiring a reverse-recovery period which could
cost as much as several percent in efficiency. A 2A to 8A
Schottky is generally a good compromise for both regions
of operation due to the relatively small average current.
Larger diodes result in additional transition loss due to
their larger junction capacitance.
C IN and C OUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V OUT )/(V IN ). To prevent
large voltage transients, a low ESR capacitor sized for the
maximum RMS current of one channel must be used. The
maximum RMS capacitor current is given by:
P SYNC = IN
V – V OUT
V IN
( I MAX ) 2 ( 1 + δ ) R DS(ON)
C IN Required I RMS ≈
I MAX
V IN
?? ( V OUT ) ( V IN – V OUT ) ??
1/2
3867f
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